What are bad normals and what they do?

[HammerFist132]

I have few models rejected because of bad models, also I heard from some guys that bad normals make model immposible to use in the unity 3d. So why they happen, how and what they do so it makes model impossible to be used, also how to fix them. I use 3ds max 2009. thank you

 

[GhostWolf]

There are two possible ways your normals can be considered "bad":

• They have wrong directions.
  Since normals ultimately define the direction a polygon faces (this effects anything related with lighting), if normals face the wrong direction, you can expect weird graphical issues to happen.
• They are inconsistent.
  If normals are not relatively consistent along the mesh, you will get all sorts of graphical artifacts.
  The best example to inconsistent normals is a cube made of 6 flat polygons, since the normals are all 90 degrees to each other, it will look bad when it comes to lighting.

As far as I remember 3ds Max has a feature to re-calculate the direction of normals and fix normals that are facing the wrong direction for some reason.
As to consistency, that's largely dependent on your mesh topology, though there are probably tools that help smooth normals out as well.

 

[Talavaj]

Normals in 3d are vectors that should be perpendicular to the face the vertex is sitting on, the information is used by interpolation shading techniques like Phong shading to define how the mesh should be illuminated.

A "bad" normal is when you get a normal that is not angled correctly and you get incorrect shading on that face.
You can't "make" a bad normal because they are generated by the 3d modeling software automatically, most commonly a bad normal in wc3 is made because the 3d model in max uses multiple normals on a vertex, but because wc3 uses per-vertex normals they get averaged into one and it ends up looking wrong.
Unity also has per-vertex normals.

Max has a number of normal editing tools, but they are useless for wc3 application seeing how no normal information is converted into wc3. (unless that has been changed in dex2, which might have been considering it now saves smoothing groups ?)
You can also use mdlvis which does a piss poor job at it and usually messes up your normals instead. (and is generally just awful, as it only allows you to manually rotate normals or average normals in one direction without thresholds)

In max you can go to edit normals and select all of them and hit "unify" that is usually how they will look like when converted into wc3.